Hitherto, in the case where a silicon single crystal substrate is etched with a chemical solution, there are a method of performing etching with an acid based etching liquid which is a mixed aqueous solution having components such as hydrofluoric acid and nitric acid, etc. added thereto; a method of performing etching with an alkali based etching liquid which is an aqueous solution of potassium hydroxide, tetramethylammonium hydroxide, etc.; and so on (see Non-Patent Documents 1 and 2).
In the case where an acid based etching liquid is used, the surface of silicon is oxidized with a component having an oxidizing action, such as nitric acid, etc., to form silicon oxide, and this silicon oxide is dissolved as silicon fluoride by hydrofluoric acid, etc., whereby etching proceeds. A characteristic feature in performing etching with an acid based etching liquid resides in the matter that even when silicon which is an etching object is monocrystalline, polycrystalline or amorphous, the etching isotropically proceeds. For that reason, in performing pattern etching using a pattern mask, etc., there may be the case where the deeper the etching is, the more the etching in a lateral direction, namely undercut (erosion) beneath the pattern mask proceeds to the same degree as the depth, resulting in causing inconvenience.
On the other hand, in the case where an alkali based etching liquid is used, silicon is dissolved as an ion of silicic acid by a hydroxy anion in the liquid, and on that occasion, water is reduced to generate hydrogen. When etching with the alkali based etching liquid is performed, different from the case of the acid based etching liquid, etching of monocrystalline silicon proceeds while keeping anisotropy. This is based on the matter that there is a difference in a dissolution rate of silicon in every crystal face orientation of silicon, and this etching is also called crystal anisotropic etching. Even in polycrystalline silicon, etching proceeds while keeping anisotropy on microscopic observations. However, in view of the fact that the face orientation of crystal grains is randomly distributed, isotropic etching appears to proceed on macroscopic observations. In amorphous silicon, etching isotropically proceeds on both of microscopic observations and macroscopic observations. As one kind of the alkali based etching liquid, there is one in which not only a difference in a dissolution rate in every crystal face orientation is utilized, but the dissolution selectivity is revealed in, as an object, a monocrystalline silicon substrate having a p-type dopant locally implanted therein depending upon the presence or absence of the dopant, and there is proposed a method of performing etching with enhanced anisotropy by the addition of an alcohol, a phenol, etc. to an alkaline aqueous solution (see Patent Document 1).
The alkaline compound to be dissolved in the alkaline etching liquid can be roughly classified into an inorganic alkaline compound such as NaOH, KOH, etc. and an organic alkaline compound such as tetramethylammonium hydroxide, choline, etc.
In the case where an inorganic alkaline aqueous solution is compared with an organic alkaline aqueous solution, in general, the inorganic alkaline aqueous solution is able to achieve a high silicon etching rate, an aspect of which, however, relies upon a condition such as concentration, temperature, etc. For that reason, in the case where it is intended to perform etching of silicon at a high etching rate, an inorganic alkaline aqueous solution is frequently used. On the contrary, in the case where there is a concern that functions of an electric device part or an electric wiring part existing on a silicon substrate in which an Na ion or a K ion is an etching object, or on other silicon substrate to be manufactured using the same equipment are influenced, an organic alkaline aqueous solution such as tetramethylammonium hydroxide (TMAH), etc. is frequently used (see Non-Patent Document 3).
Also, besides the etching application of silicon, TMAH is a compound which is generally used in the field of electronic materials, in particular, the field of cleaning solvents, etc. For example, Patent Document 2 discloses the use of a cleaning agent composed of TMAH and a guanidine salt as a water-soluble detergent for removing contaminants (resists and dry etching residues, etc.) adhered to a substrate such as silicon, a metal, an interlayer dielectric material, etc. without etching the substrate itself; however, this patent document is quite different from the present invention which is aimed to perform etching of silicon itself.    [Patent Document 1] JP-A-6-188236    [Patent Document 2] JP-A-2007-16232    [Non-Patent Document 1] Sato, “Silicon Etching Technologies” in Journal of the Surface Finishing Society of Japan, Vol. 51, No. 8, 2000, pages 754 to 759    [Non-Patent Document 2] Esashi, 2003 MEMS Technology Outlook, pages 109 to 114    [Non-Patent Document 3] Tabata, “Silicon. Crystal Anisotropic Etching Using TMAH Solution” in Journal of the Surface Finishing Society of Japan, Vol. 51, No. 8, 2000, pages 767 to 772